On both installs, Safari 18.4 is not working. When I try to go anywhere on the Web, I get an error saying "A problem repeatedly occured with ....(URL)"
Firefox (latest download) is working fine.
Is that a known issue with a 3,1?
Also with any other Mac that does not have AVX/AVX2 support - Safari JIT now requires AVX support, did you try disabling it?
P.S.:
Disabling is setting it to false.